Nuclear Fission

World of Earth Science
COPYRIGHT 2003 The Gale Group, Inc.

Nuclear fission

Nuclear fission is a process in which the nucleus of an atom splits, usually into two daughter nuclei. Spontaneous fission of uranium and other elements in Earth's interior provides an internal source of heat that drives plate tectonics . Fission tracks in mineral crystals , a result of spontaneous fission of uranium, can be used in radiometric dating of rock and rock layers.

Long before the internal construction of the atom was well understood in terms of protons, neutrons, and electrons, nuclear transformations that resulted in observable radioactivity were observed as early as 1896 by French physicist Henri Becquerel (1852–1908). In 1905, British physicist Ernst Rutherford (1871–1937) and American physicist Bertram Borden Boltwood (1870–1927) first used radioactive decay measurements to date minerals .

The fission reaction was discovered when a target of uranium was bombarded by neutrons. Fission fragments were shown to fly apart with a large release of energy. The fission reaction was the basis of the atomic bomb first developed by the United States during World War II. After the war, controlled energy release from fission was applied to the development of nuclear reactors. Reactors are utilized for production of electricity at nuclear power plants, for propulsion of ships and submarines, and for the creation of radioactive isotopes used in medicine and industry.

The fission reaction was first articulated by two German scientists, Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980). In 1938, Hahn and Strassmann conducted a series of experiments in which they used neutrons to bombard various elements. Bombardment of copper, for example, produced a radioactive form of copper. Other elements became radioactive in the same way. When uranium was bombarded with neutrons, however, an entirely different reaction occurred. The uranium nucleus apparently underwent a major disruption. Accordingly, the initial evidence for the fission process came from chemical analysis. Hahn and Strassmann published a scientific paper showing that small amounts of barium (element 56) were produced when uranium (element 92) was bombarded with neutrons. Hahn and Strassmann questioned how a single neutron could transform element 92 into element 56.

Lise Meitner (1878–1968), a long-time colleague of Hahn who had left Germany due to Nazi persecution, suggested a helpful model for such a reaction. One can visualize the uranium nucleus to be like a liquid drop containing protons and neutrons. When an extra neutron enters, the drop begins to vibrate. If the vibration is violent enough, the drop can break into two pieces. Meitner named this process "fission" because it is similar to the process of cell division in biology. Moreover, it takes only a relatively small amount of energy to initiate nuclear instability.

Scientists in the United States and elsewhere quickly confirmed the idea of uranium fission, using other experimental procedures. For example, a cloud chamber is a device in which vapor trails of moving nuclear particles can be seen and photographed. In one experiment, a thin sheet of uranium was placed inside a cloud chamber. When it was irradiated by neutrons, photographs showed a pair of tracks going in opposite directions from a common starting point in the uranium. Clearly, a nucleus had been photographed in the act of fission.

Another experimental procedure used a Geiger counter, which is a small, cylindrical tube that produces electrical pulses when a radioactive particle passes through it. For this experiment, the inside of a modified Geiger tube was lined with a thin layer of uranium. When a neutron source was brought near it, large voltage pulses were observed, much larger than from ordinary radioactivity. When the neutron source was taken away, the large pulses stopped. A Geiger tube without the uranium lining did not generate large pulses. Evidently, the large pulses were due to uranium fission fragments. The size of the pulses showed that the fragments had a very large amount of energy.

To understand the high energy released in uranium fission, scientists made some theoretical calculations based on German-American physicist Albert Einstein's (1879–1955) famous equation E=mc2. The Einstein equation states that mass (m ) can be converted into energy (E ) (and, conversely that energy can create mass). The conversion factor becomes c, the velocity of light squared. One can calculate that the total mass of the fission products remaining at the end of the reaction is slightly less than the mass of the uranium atom plus neutron at the start. This decrease of mass, multiplied by c, shows numerically why the fission fragments are so energetic.

Through fission, neutrons of low energy can trigger off a very large energy release. With the imminent threat of war in 1939, a number of scientists began to consider the possibility that a new and very powerful "atomic bomb" could be built from uranium. Also, they speculated that uranium perhaps could be harnessed to replace coal or oil as a fuel for industrial power plants.

Nuclear reactions in general are much more powerful than chemical reactions. A chemical change such as burning coal or even exploding TNT affects only the outer electrons of an atom. A nuclear process, on the other hand, causes changes among the protons and neutrons inside the nucleus. The energy of attraction between protons and neutrons is about a million times greater than the chemical binding energy between atoms. Therefore, a single fission bomb, using nuclear energy, might destroy a whole city. Alternatively, nuclear electric power plants theoretically could run for a whole year on just a few tons of fuel.

In order to release a substantial amount of energy, many millions of uranium nuclei must split apart. The fission process itself provides a mechanism for creating a so-called chain reaction. In addition to the two main fragments, each fission event produces two or three extra neutrons. Some of these can enter nearby uranium nuclei and cause them in turn to fission, releasing more neutrons, which causes more fission, and so forth. In a bomb explosion, neutrons have to increase very rapidly, in a fraction of a second. In a controlled reactor, however, the neutron population has to be kept in a steady state. Excess neutrons must be removed by some type of absorber material (e.g., neutron absorbing control rods).

In 1942, the first nuclear reactor with a self-sustaining chain reaction was built in the United States. The principal designer was Enrico Fermi (1901–1954), an Italian physicist and the 1938 Nobel Prize winner in physics. Fermi emigrated to the United States to escape from Benito Mussolini's fascism. Fermi's reactor design had three main components: lumps of uranium (the fuel), blocks of carbon (the moderator, which slows down the neutrons), and control rods made of cadmium (an excellent neutron absorber). Fermi and other scientists constructed the first nuclear reactor pile at the University of Chicago. When the pile of uranium and carbon blocks was about 10 ft (3 m) high and the cadmium control rods were pulled out far enough, Geiger counters showed that a steady-state chain reaction had been successfully accomplished. The power output was only about 200 watts, but it was enough to verify the basic principle of reactor operation. The power level of the chain reaction could be varied by moving the control rods in or out.

General Leslie R. Groves was put in charge of the project to convert the chain reaction experiment into a usable military weapon. Three major laboratories were built under wartime conditions of urgency and secrecy. Oak Ridge, Tennessee, became the site for purifying and separating uranium into bomb-grade material. At Hanford, Washington, four large reactors were built to produce another possible bomb material, plutonium. At Los Alamos, New Mexico, the actual work of bomb design was started in 1943 under the leadership of the physicist J. Robert Oppenheimer (1904–1967).

The fissionable uranium isotope, uranium-235, constitutes only about l% of natural uranium, while the non-fissionable neutron absorber, uranium-238, makes up the other 99%. To produce bomb-grade, fissionable uranium-235, it was necessary to build a large isotope separation facility. Since the plant would require much electricity, the site was chosen to be in the region of the Tennessee Valley Authority (TVA). The technology of large-scale isotope separation involved solving many difficult, unprecedented problems. By early 1945, the Oak Ridge Laboratory was able to produce kilogram amounts of uranium-235 purified to better than 95%.

An alternate possible fuel for a fission bomb is plutonium-239. Plutonium does not exist in nature but results from radioactive decay of uranium-239. Fermi's chain reaction experiment had shown that uranium-239 could be made in a reactor. However, to produce several hundred kilograms of plutonium required a large increase from the power level of Fermi's original experiment. Plutonium production reactors were constructed at Hanford, Washington, located near the Columbia river to provide needed cooling water . A difficult technical problem was how to separate plutonium from the highly radioactive fuel rods after irradiation. This was accomplished by means of remote handling apparatus that was manipulated by technicians working behind thick protective glass windows.

With uranium-235 separation started at Oak Ridge and plutonium-239 production under way at Hanford, a third laboratory was set up at Los Alamos, New Mexico, to work on bomb design. In order to create an explosion, many nuclei would have to fission almost simultaneously. The key concept was to bring together several pieces of fissionable material into a so-called critical mass. In one design, two pieces of uranium-235 were shot toward each other from opposite ends of a cylindrical tube. A second design used a spherical shell of plutonium-239, to be detonated by an "implosion" toward the center of the sphere.

The first atomic bomb was tested at an isolated desert location in New Mexico on July 16, 1945. President Truman then issued an ultimatum to Japan that a powerful new weapon could soon be used against them. On August 8, a single atomic bomb destroyed the city of Hiroshima with over 80,000 casualties. On August 11, a second bomb was dropped on Nagasaki with a similar result. Japan surrendered three days later to end WWII.

The decision to use the atomic bomb has been vigorously debated over the years. It ended the war and avoided many casualties that a land invasion of Japan would have cost. However, the civilians who were killed by the bomb and the survivors who developed radiation sickness left an unforget-table legacy of fear. The horror of mass annihilation in a nuclear war is made vivid by the images of destruction at Hiroshima. The possibility of a ruthless dictator or a terrorist group obtaining nuclear weapons is a continuing threat to world peace. In late 2001, in the aftermath of the terrorist attacks on the World Trade Center in New York, evidence became public of terrorist attempts to acquire weapons-grade uranium and the other technology related to bomb production.

The first nuclear reactor designed for producing electricity was put into operation in 1957 at Shippingsport, Pennsylvania. From 1960 to 1990, more than 100 nuclear power plants were built in the United States. These plants now generate about 20% of the nation's electric power. Worldwide, there are over 400 nuclear power stations.

The most common reactor type is the pressurized water reactor (abbreviated PWR). The system operates like a coal-burning power plant, except that the firebox of the coal plant is replaced by a reactor. Nuclear energy from uranium is released in the two fission fragments. The fuel rod becomes very hot because of the cumulative energy of fissioning nuclei. A typical reactor core contains hundreds of these fuel rods.

Water is circulated through the core to remove the heat. The hot water is prevented from boiling by keeping the system under pressure (i.e., creating superheated steam).

The pressurized hot water goes to a heat exchanger where steam is produced. The steam then goes to a turbine, which has a series of fan blades that rotate rapidly when hit by the steam. The turbine is connected to the rotor of an electric generator. Its output goes to cross-country transmission lines that supply the electrical users in the region with electricity. The steam that made the turbine rotate is condensed back into water and is recycled to the heat exchanger.

Safety features at a nuclear power plant include automatic shutdown of the fission process by insertion of control rods, emergency water-cooling for the core in case of pipeline breakage, and a concrete containment shell. It is impossible for a reactor to have a nuclear explosion because the fuel enrichment in a reactor is intentionally limited to about 3% uranium-235, while almost 100% pure uranium-235 is required for a bomb. Regardless, nuclear power plants remain potential targets for terrorists who would seek to cause massive and lethal release of radioactivity by compromising the containment shell.

The fuel in the reactor core consists of several tons of uranium. As the reactor is operated, the uranium content gradually decreases because of fission, and the radioactive waste products (the fission fragments) build up. After about a year of operation, the reactor must be shut down for refueling. The old fuel rods are pulled out and replaced. These fuel rods, which are very radioactive, are stored under water at the power plant site. After five to 10 years, much of their radioactivity has decayed. Only those materials with a long radioactive lifetime remain, and eventually they will be stored in a suitable under-ground depository.

There are vehement arguments for and against nuclear power. As with other forms of producing electricity, nuclear power generation can have serious and unintended environmental impacts. The main objections to nuclear power plants are the fear of possible accidents, the unresolved problem of nuclear waste storage, and the possibility of plutonium diversion for weapons production by a terrorist group. The issue of waste storage becomes particularly emotional because leakage from a waste depository could contaminate ground water. Opponents of nuclear power often cite accidents at the Three Mile Island nuclear power plant in the United States and the massive leak at the Chernobyl nuclear plant in the USSR (now the Ukraine) as evidence that engineering or technical failures can have long lasting and devastating environmental and public health consequences.

The main advantage of nuclear power plants is that they do not cause atmospheric pollution . No smokestacks are needed because nothing is being burned. France initiated a large-scale nuclear program after the Arab oil embargo in 1973 and has been able to reduce its acid rain and carbon dioxide emissions by more than 40%. Nuclear power plants do not contribute to global warming . Shipments of fuel are minimal so the hazards of coal transportation and oil spills are avoided.

Environmentalists remain divided in their opinions of nuclear power. It is widely viewed as a hazardous technology but there is growing concern about atmospheric pollution and dwindling fossil fuel reserves that may make increased usage of nuclear power more widespread.

Nuclear Fission

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Nuclear fission

Nuclear fission is a process in which the nucleus of a heavy atom is broken apart into two or more smaller nuclei. The reaction was first discovered in the late 1930s when a target of uranium metal was bombarded with neutrons. Uranium nuclei broke into two smaller nuclei of roughly equal size with the emission of very large amounts of energy. Some scientists immediately recognized the potential of the nuclear fission reaction for the production of bombs and other types of weapons as well as for the generation of power for peacetime uses.

History

The fission reaction was discovered accidentally in 1938 by two German physicists, Otto Hahn (1879–1968) and Fritz Strassmann (1902–1980). Hahn and Strassmann had been doing a series of experiments in which they used neutrons to bombard various elements. When they bombarded copper, for example, a radioactive form of copper was produced. Other elements became radioactive in the same way.

Their work with uranium, however, produced entirely different results. In fact, the results were so unexpected that Hahn and Strassmann were unable to offer a satisfactory explanation for what they observed. That explanation was provided, instead, by German physicist Lise Meitner (1878–1968) and her nephew Otto Frisch (1904–1979). Meitner was a longtime colleague of Hahn who had left Germany due to anti-Jewish persecution.

In most nuclear reactions, an atom changes from a stable form to a radioactive form, or it changes to a slightly heavier or a slightly lighter atom. Copper (element number 29), for example, might change from a stable form to a radioactive form or to zinc (element number 30) or nickel (element number 28). Such reactions were already familiar to nuclear scientists.

What Hahn and Strassmann had seen—and what they had failed to recognize—was a much more dramatic nuclear change. An atom of uranium (element number 92), when struck by a neutron, broke into two much smaller elements such as krypton (element number 36) and barium (element number 56). The reaction was given the name nuclear fission because of its similarity to the process by which a cell breaks into two parts during the process of cellular fission.

Putting nuclear fission to work

In every nuclear fission, three kinds of products are formed. The first product consists of the smaller nuclei produced during fission. These nuclei, like krypton and barium in the example mentioned above, are called fission products. Fission products are of interest for many reasons, one of which is that they are always radioactive. That is, any time a fission reaction takes place, radioactive materials are formed as by-products of the reaction.

Words to Know

Chain reaction: A reaction in which a substance needed to initiate a reaction is also produced as the result of that reaction.

Fission products: The isotopes formed as the result of a nuclear fission reaction.

Fission weapon: A bomb or other type of military weapon whose power is derived from a nuclear fission reaction.

Isotopes: Two or more forms of an element that have the same chemical properties but that differ in mass because of differences in the number of neutrons in their nuclei.

Manhattan Project: A research project of the United States government created to develop and produce the world's first atomic bomb.

Mass: A measure of the amount of matter in a body.

Neutron: A subatomic particle with a mass about equal to that of a hydrogen atom but with no electric charge.

Nuclear reactor: Any device for controlling the release of nuclear power so that it can be used for constructive purposes.

Radioactivity: The property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.

Radioactive isotope: An isotope that spontaneously breaks down into another isotope with the release of some form of radiation.

The second product of a fission reaction is energy. A tiny amount of matter in the original uranium atom is changed into energy. In the early 1900s, German-born American physicist Albert Einstein (1879–1955) had showed how matter and energy can be considered two forms of the same phenomenon. The mathematical equation that represents this relationship, E = mc2, has become one of the most famous scientific formulas in the world. The formula says that the amount of energy (E) that can be obtained from a certain amount of matter (m) can be found by multiplying that amount of matter by the square of the speed of light (c2). The square of the speed of light is a very large number, equal to about 9 × 1020 meters per second, or 900,000,000,000,000,000,000 meters per second. Thus, if even a very small amount of matter is converted to energy, the amount of energy obtained is very large. It is this availability of huge amounts of energy that originally made the fission reaction so interesting to both scientists and nonscientists.

The third product formed in any fission reaction is neutrons. The significance of this point can be seen if you recall that a fission reaction is initiated when a neutron strikes a uranium nucleus or other large nucleus. Thus, the particle needed to originate a fission reaction is also produced as a result of the reaction.

Chain reactions. Imagine a chunk of uranium metal consisting of trillions upon trillions of uranium atoms. Then imagine that a single neutron is fired into the chunk of uranium, as shown in the accompanying figure of a nuclear chain reaction. If that neutron strikes a uranium nucleus, it can cause a fission reaction in which two fission products and two neutrons are formed. Each of these two neutrons, in turn, has the potential for causing the fission of two other uranium nuclei. Two neutrons produced in each of those two reactions can then cause fission in four uranium nuclei. And so on.

In actual practice, this series of reactions, called a chain reaction, takes place very rapidly. Millions of fission reactions can occur in much less than a second. Since energy is produced during each reaction, the total amount of energy produced throughout the whole chunk of uranium metal is very large indeed.

The first atomic bomb

Perhaps you can see why some scientists immediately saw fission as a way of making very powerful bombs. All you have to do is to find a large enough chunk of uranium metal, bombard the uranium with neutrons, and get out of the way. Fission reactions occur trillions of times over again in a short period of time, huge amounts of energy are released, and the uranium blows apart, destroying everything in its path. Pictures of actual atomic bomb blasts vividly illustrate the power of fission reactions.

But the pathway from the Hahn/Strassmann/Meitner/Frisch discovery to an actual bomb was a long and difficult one. A great many technical problems had to be solved in order to produce a bomb that worked on the principle of nuclear fission. One of the most difficult of those problems involved the separation of uranium-238 from uranium-235.

Naturally occurring uranium consists of two isotopes: uranium-238 and uranium-235. Isotopes are two forms of the same element that have the same chemical properties but different masses. The difference between these two isotopes of uranium is that uranium-235 nuclei will undergo nuclear fission, but those of uranium-238 will not. That problem is compounded by the fact that uranium-238 is much more abundant in nature than is uranium-235. For every 1,000 atoms of uranium found in Earth's crust, 993 are atoms of uranium-238 and only 7 are atoms of uranium-235. One of the biggest problems in making fission weapons a reality, then, was finding a way to separate uranium-235 (which could be used to make bombs) from uranium-238 (which could not, and thus just got in the way).

The Manhattan Project. A year into World War II (1939–45), a number of scientists had come to the conclusion that the United States would have to try building a fission bomb. They believed that Nazi Germany would soon be able to do so, and the free world could not survive unless it, too, developed fission weapons technology.

Thus, in 1942, President Franklin D. Roosevelt authorized the creation of one of the largest and most secret research operations ever devised. The project was given the code name Manhattan Engineering District, and its task was to build the world's first fission (atomic) bomb. That story is a long and fascinating one, a testimony to the technological miracles that can be produced under the pressures of war. The project reached its goal on July 16, 1945, in a remote part of the New Mexico desert, where the first atomic bomb was tested. Less than a month later, the first fission bomb was actually used in war. It was dropped on the Japanese city of Hiroshima, destroying the city and killing over 80,000 people. Three days later, a second bomb was dropped on Nagasaki, with similar results. For all the horror they caused, the bombs seemed to have achieved their objective. The Japanese leaders appealed for peace only three days after the Nagasaki event. (Critics, however, charge that the end of the war was in sight and that the Japanese would have surrendered without the use of a devastating nuclear weapon.)

Nuclear fission in peacetime

The world first learned about the power of nuclear fission in the form of terribly destructive weapons, the atomic bombs. But scientists had long known that the same energy released in a nuclear weapon could be harnessed for peacetime uses. The task is considerably more difficult, however. In a nuclear weapon, a chain reaction is initiated—energy is produced and released directly to the environment. In a nuclear power reactor, however, some means must be used to control the energy produced in the chain reaction.

The control of nuclear fission energy was actually achieved before the production of the first atomic bomb. In 1942, a Manhattan Project research team under the direction of Italian physicist Enrico Fermi (1901–1954) designed and built the first nuclear reactor. A nuclear reactor is a device for obtaining the controlled release of nuclear energy. The reactor had actually been built as a research instrument to learn more about nuclear fission (as a step in building the atomic bomb).

After the war, the principles of Fermi's nuclear reactor were used to construct the world's first nuclear power plants. These plants use the

energy released by nuclear fission to heat water in boilers. The steam that is produced is then used to operate turbines and electrical generators. The first of these nuclear power plants was constructed in Shippingport, Pennsylvania, in 1957. In the following three decades, over 100 more nuclear power plants were built in every part of the United States, and at least as many more were constructed throughout the world.

By the dawn of the 1990s, however, progress in nuclear power production had essentially come to a stop in the United States. Questions about the safety of nuclear power plants had not been answered to the satisfaction of most Americans, and, as a result, no new nuclear plants have been built in the United States since the mid-1980s.

Despite these concerns, nuclear power plants continue to supply a good portion of the nation's electricity. Since 1976, nuclear electrical generation has more than tripled. At the beginning of the twenty-first century, 104 commercial nuclear power reactors in 31 states accounted for about 22 percent of the total electricity generated in the country. Combined, coal and nuclear sources produce 78 percent of the nation's electricity.

[See alsoNuclear fusion; Nuclear power; Nuclear weapons ]

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Nuclear Fission

Nuclear Fission

Following the discovery of the neutron in the early 1930s, nuclear physicists began bombarding a variety of elements with neutrons. Enrico Fermi in Italy included uranium (atomic number 92) among the elements he bombarded which resulted in formation of nuclei that decayed by emission of negative β -rays. Such decay produces nuclei of higher atomic number, so Fermi assumed that the bombardment of uranium led to a new element with atomic number 93. By 1938 similar research had resulted in reports of the discovery of four new elements with atomic numbers 93, 94, 95, and 96. In 1938 Otto Hahn and Fritz Strassmann in Berlin bombarded uranium with neutrons to study the possibility of production of nuclides with atomic numbers less then 92 due to emission of protons and α -particles. To their surprise, they found that they had made barium (z = 56). Hahn informed a former colleague, Lise Meitner, who, with Otto Frisch, reviewed the data and reached the conclusion that the uranium atom was splitting (fissioning) into two new, smaller nuclei with the accompanying release of a large amount of energy. Many laboratories quickly confirmed the occurrence of this process of nuclear fission. Niels Bohr and John Wheeler, within a few months, published a paper explaining many features of fission using a model of nuclear behavior based on an analogy to a droplet of liquid, which, when given extra energy, can elongate from a spherical shape and split into two smaller droplets. Nuclei have two opposing energies: a disruptive energy resulting from the mutual electrostatic repulsion of the positive protons in the nucleus, and an attractive energy due to nuclear forces present between the nuclear particles (both neutrons and protons). The repulsive electrostatic energy of the protons increases as the number of protons increases and decreases as the average distance between them increases. The attractive nuclear

force energy increases with the total number of nucleons (protons and neutrons) in the nucleus. The nuclear attractive force is at a maximum when the nucleus has a spherical shape and at a minimum when the nucleus is distorted into two roughly equal fragments.

Nuclei formed in fission, known as fission products, range in atomic number (number of protons) from approximately 30 to 64. The original fissioning nuclide has a neutron to proton ration of about 1.6, whereas stable nuclei having the same range of atomic numbers as the fission products have neutron to proton ratios of 1.3 to 1.4. This means that nuclei formed in fission have too great a number of neutrons for stability and undergo beta (β−) decay to convert neutrons to protons. In general, fission is restricted to nuclei with atomic numbers above 82 (Pb), and the probability of fission increases as the atomic number increases. Fission produces nuclei of atomic masses from above 60 to about 150.

With very-low-energy neutrons, uranium of mass number 235 emits an average of two to three neutrons per fission event. Because more neutrons are released than absorbed, fission can result in a multiplication of successive fission events. This multiplication can reach very high numbers in about 10−14 to 10−17 seconds, resulting in the release of a great amount of energy in that time. This was the basis of the development of nuclear weapons. Soon after the discovery of fission it had been calculated that if a sufficient quantity of the fissionable material was assembled under proper conditions, a self-sustaining nuclear explosion could result. The critical mass of the fissionable material necessary for explosion is obtained with a spherical shape (minimum surface area per mass). The uranium isotope of mass 235 and the plutonium isotope of mass 239 are incited to fission and release energy in the use of nuclear weapons and in nuclear reactors. Nuclear reactors control the rate of fission and maintain it at a constant level, allowing the released energy to be used for power. Nuclear reactors are used in many nations as a major component of their natural energy. In the United States, approximately 20 percent of the electricity is provided by nuclear reactors, whereas France uses reactors to produce almost 80 percent of its electricity. Reactors used for power have four basic components: (1) fuel, either natural uranium or uranium enriched in 235U or 239Pu; (2) a moderator to reduce neutron energies, which increases the probability of fission; (3) control rods of cadmium and boron to control the rate of fission; and (4) coolants to keep the temperature of the reactor at a reasonable level and to transfer the energy for production of electricity. In power reactors the coolant is commonly H2O or D2O, but air, graphite, or a molten mixture of sodium and potassium can be used. Reactors are surrounded by a thick outer shield of concrete to prevent release of radiation.

There have been two major accidents (Three Mile Island in the United States and Chernobyl in the former Soviet Union) in which control was lost in nuclear power plants, with subsequent rapid increases in fission rates that resulted in steam explosions and releases of radioactivity. The protective shield of reinforced concrete, which surrounded the Three Mile Island Reactor, prevented release of any radioactivity into the environment. In the Russian accident there had been no containment shield, and, when the steam explosion occurred, fission products plus uranium were released to the environment—in the immediate vicinity and then carried over the Northern Hemisphere, in particular over large areas of Eastern Europe. Much was learned from these accidents and the new generations of reactors are being built to be "passive" safe. In such "passive" reactors, when the power level increases toward an unsafe level, the reactor turns off automatically to prevent the high-energy release that would cause the explosive release of radioactivity. Such a design is assumed to remove a major factor of safety concern in reactor operation.

LEO SZILARD (1898–1964)

Leo Szilard determined that the formation of neutrons occurs during the fission of uranium. This is crucial to sustaining a chain reaction necessary to build an atomic bomb, the first of which he helped to construct in 1942. Shortly thereafter, realizing the destructive power of the atom bomb, Szilard argued for an end to nuclear weapons research.

fission, nuclear

fission, nuclear In physics, form of nuclear reaction in which a heavy atomic nucleus splits into two, with the release of two or three neutrons and large amounts of heat and nuclear radiation. Fission may occur spontaneously or be made to occur by bombarding certain nuclei with low-energy (slow) neutrons or protons. The only naturally occurring fissionable material is uranium-235. Natural uranium is not capable of sustaining a chain reaction because its major isotope, U238, tends to absorb neutrons before they can split another atom. Currently, there are two main methods of creating a sustained chain reaction in uranium fuel. In the first, the neutrons released by fission of U235 are slowed down by a moderator composed of light atoms, such as deuterium, which does not absorb neutrons. Most nuclear reactors use this method. Alternatively, the uranium may be enriched (increasing the amount of fissionable material). Fast-breeder reactors employ this process. Nuclear fission and nuclear fusion are used in nuclear weapons.

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